A fuel assembly of a known design comprises a plurality of fuel rods and guide thimbles for control rods or for coolant flowing through. The fuel rods and the guide thimbles are retained in predetermined mutual positions by means of a plurality of spacer grids which are arranged at different levels along the fuel assembly. A top and a bottom nozzle are arranged at the respective ends of the fuel assembly for attachment of the guide thimbles. Attachment of the guide thimbles to the top and bottom nozzles is make possible since the guide thimbles extend somewhat outside the respective ends of the fuel rods. In Swedish patent 465 796 such a fuel assembly is described. According to the patent, the guide thimbles are provided at the top with a top sleeve which fits into a corresponding hole arranged through the top nozzle. Inside the top sleeve a guide sleeve is arranged which is insertable through that hole in the top nozzle which corresponds to the respective top sleeve. A locking sleeve is, in turn, arranged insertable into the guide sleeve for interlocking thereof in the top sleeve in connected position, that is, when the top nozzle is fixedly interlocked with the guide thimbles by means of the locking device.
The inside of the top sleeve is provided with an annular slot for cooperation with an outer bead arranged in the guide sleeve. In that end of the guide sleeve which faces the top sleeve, an axially extending slit is arranged and intended to allow compression of the guide sleeve into a diameter which allows inserting the bead into the internal slot of the top sleeve. For reliable interlocking of the locking sleeve to the guide sleeve, the locking sleeve, after insertion into the guide sleeve, may be provided with a bulge for cooperation with the rear side of the outer bead of the guide sleeve; alternatively, the locking sleeve is provided with bulges below the lower end of the guide sleeve, which lower end is arranged in the fuel assembly.
The disadvantage of the arrangement described above is that in those cases where the locking sleeve is not bulged for reliable interlocking, there is a risk that it gets out of position when inserting and withdrawing the control-rod guide thimbles. In those cases where the locking sleeve is provided with a bulge, the bulge is provided after the locking sleeve has become inserted into the guide sleeve, which means that the dimension of the bulge cannot be determined with certainty, and hence nor the interlocking force of the locking sleeve in the guide sleeve. In those cases where the locking sleeve is provided with bulges, a detached locking sleeve and guide sleeve, owing to the damage which arises during the detachment, must be discarded. A further disadvantage is that the location of the bulges described above at the lower part of the guide sleeve means that the locking sleeve must be drawn a relatively long distance in the axial direction to be released from the engagement with the guide sleeve.